However, the map layout is data, not branding. If your service has to alter the layout, it's more confusing to the passengers who TfL should be thinking about.

It has a very unique look and feel, and I'm not sure it is "just" data given that the location of stations on the tube map doesn't actually correspond to their geographic locations within London. I do think it is capable of forming part of the TfL brand, though by now it feels quite generic to me.

Regardless of whether the map is capable of being protected by IP law (TfL certainly seem to think it is), this just feels stingy and pointless on TfL's part. They are a public service after all, and these maps arguably furthered their public mission. Given how popular the map is I would much prefer they published it under a licence allowing free non-commercial use with attribution (including a statement that the user is not affiliated with TfL).